Also, the Xenomorphs never really needed a purpose or a full origin. They were animals (surviving is their purpose) that are from an alien planet. Asking for the purpose of a xenomorph was like asking for the purpose of a dog. It's a dog. That's it's life goal. Be a dog.
NAH UH!... A Dog goal in life is to help me be the best, so I gotta catch them all!... Joke aside. I'll still take a crack at it, heard its better then most movies that are out at the moment.